Under pressure from gun control advocates, Facebook has agreed to delete posts from users selling illegal guns or offering weapons for sale without background checks.
A similar policy will be applied to Instagram, the company's photo-sharing network, Facebook said.
The measures will be put into effect over the next few weeks and will apply worldwide at Facebook, which claims 1.3 billion active users.
"We will remove reported posts that explicitly indicate a specific attempt to evade or help others evade the law," the company said in a statement on Wednesday.
New York Attorney General Eric Schneiderman has been asking Facebook to adopt such restrictions, as have Mayors Against Illegal Guns, the group backed by former New York Mayor Michael Bloomberg, and Moms Demand Action.
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Moms Demand Action collected more than 230,000 signatures on petitions calling on Facebook to act.
"Responsible social media sites know that it is in no one's interest for their sites to become the 21st-century black market in dangerous and illegal goods that place our families and communities at risk," Schneiderman said.
"I congratulate Facebook and Instagram for taking these simple, commonsense steps to protect the safety and security of their users, and encourage other social media sites to follow their lead."
Tom King, president of the New York affiliate of the National Rifle Association, acknowledged that Facebook is allowed to set its own rules but said he regards the new restrictions as "a kind of limit on our First Amendment rights".
